The present invention relates to fibers, such as melt-extruded fibers, and to nonwoven webs prepared therefrom.
Air filter materials may be improved by treating the nonwovens in the presence of a high-intensity external electric field, thereby endowing the web with local electric fields which persist even after the high intensity electric field is removed (electret treatment). The electric fields associated with the fibers of the web can be used to attract foreign particles from a fluid stream which typically is air; i.e., the treatment imparts to the web an additional mechanism--attraction via electric field--beyond physical entrapment, to filter out foreign particles.
The use of electrically charged fibrous materials as filtration media has been known for some time. The advantage of materials of this type is that the charge on the fibers considerably augments the filtration efficiency without making any contribution to the airflow resistance. Among various dust filters, those made of electret fibers have high dust removing performances and are therefore suitable for attaining a high degree of cleanliness.
It is known that certain dielectric materials can be permanently electrostatically polarized, such as by heating the material, applying a high-voltage electric field, and cooling the material while under the influence of the electric field. Upon the removal of the electric field, an appropriate dielectric material becomes the electrical equivalent of a permanent magnet. A dielectric becomes an electret when the rate of decay of the field-induced polarization can be slowed down so much that a significant fraction of the polarization is preserved long after the polarizing field has been removed. Such electrets can be created by various methods, e.g. corona charging, triboelectric charging (friction), or any other charging technique (e.g. by liquid contact).
It has been established that air filters made of electret fibers are very effective in removing submicron aerosols. The electrostatic collection mechanism increases the efficiency of these electrostatically charged fibrous nonwoven materials relative to conventional, uncharged fibers. The filters have an increased ability for the capture of particles with no corresponding increase in pressure drop. Dust filters have been made from films prepared from nonpolar polymeric materials in which the films are drawn, corona-charged, and treated with needle rolls to make fibrous materials which are then formed into the filters. Alternatively, a nonwoven fabric made of polypropylene fibers and rayon fibers may be subjected to resin processing, followed by bending or shearing, whereby the surface layer of the fabric is charged with static electricity.
Electret formation may involve disposing a thread or filaments in an electro-static field established between parallel closely spaced electrodes. Alternatively, a monofilament fiber, such as a polypropylene fiber, is closely wound on a hollow winding roller which has been previously surfaced with a polyamide-faced aluminum foil. This process, however, is discontinuous and requires charging times in excess of three hours for the wrapped roll.
Other processes for forming electrets involve softening the fibers in thermoplastic polymer webs with heat and, while the fibers are soft, subjecting them to a suitable electrostatic field to produce a charged fibrous web. This technique may be carried out with a film which then is fibrillated to form fibers which are collected and formed into a filter. An electrostatic spinning process is known in which a fibrous material is sprayed electrostatically from a liquid state and deposited on a conductive support. Meltblown fibers may be charged after being formed and before being deposited to form a web.
Several cold charging processes for the preparation of charged webs are known. Examples include the corona charging of combined webs made from layers of materials with differing conductivities. Charging is accompanied by utilizing a contact web, which is more conductive than the dielectric fibers of the filtration medium, and applying the charge through the more conductive medium. Another process involves placing a nonconductive web between the surface of a grounded metal electrode and a series of discharge electrodes. A suitable web (or film) may be conveniently cold charged by sequentially subjecting the web (or film) to a series of electric fields such that adjacent electric fields have substantially opposite polarities with respect to each other. In another method, a polymer film initially is passed across a corona discharge which imparts positive and negative charges on opposite sides of the film. The film then is mechanically split into small filaments, which are subsequently formed into a filter mat. In yet another process, a charge is released between fine wires and a surface electrode. The wires are biased with an electrostatic potential of several kilovolts. The structure to be charged, be it fiber or fabric, is positioned between the electrodes. Stable ions have been implanted, in the presence of a strong electric field, into the fibers of a polymeric filter structure which is at a temperature above the glass transition temperature but below the melt temperature of the polymer.
Triboelectric charging involves bringing two or more polymers into close contact and, due to their different dielectric properties, charge is transferred from one to the other. After taking both polymers apart, they are left in a charged state. In a variation, the fibers of a filter mat are coated with particles of zinc colophony resin. The fibrous structure is mechanically needled to fracture the zinc resin crystals. The frictional effect of particle-to-particle attrition and/or crystal fracture along internal planes is sufficient to cause the particles to acquire a positive or negative charge.
Many types of polymers have been investigated for use as air filters made of electret fibers. Suitable polymers for electrets are polyolefins (e.g., polypropylene and polyethylene), polycondensates (e.g., polyamides, polyesters, polycarbonates, and polyarylates), polyacrylates, polyacetals, polyimides, cellulose esters, polystyrenes, fluoropolymers, and polyphenylenesulfide. Also suitable are combinations of polymers (e.g., copolymers and polymer blends).
It is known that certain additives improve the efficiency of electret performance, but with sometimes variable results. Examples of additives or additive/polymer combinations include titanium dioxide in polyacrylate, a fatty acid metal salt (such as magnesium stearate and aluminum palmitate) in an insulating polymer material (e.g., polypropylene, polyethylene, polyesters, and polyamides). Other additives include charge control agents, such as those employed in toners for electrophotographic processes. These agents have been blended with polyolefins and other polymers. Organic or organometallic charge control agents have been used in aromatic polyamides, polyolefins, and polyesters.
Such materials as organic acids that are solids at room temperature, inorganic materials (e.g., ceramics, metal nitrides, and carbon black), and metallic materials (e.g., silver, copper, aluminum, and tin), have been attached to the surfaces of structures to be electrified. In a variation, the surfaces of fibrous webs have been subjected to a blast of a particle-containing aerosol or to metallic vapor deposition so as to provide solid discontinuous particles at the surfaces. The webs then are electrified.
Most of the known polymeric electrets are composed solely of a nonpolar or polar polymeric material or binary electrets comprising a nonpolar polymer and a polar polymer. Binary electrets, comprising both types of polymers, have been developed and produced so as to utilize the merits of both the polar and nonpolar polymers and provide electrets retaining the excellent characteristics of both the polymers. It is known that a blend system, in which a nonpolar polymer is a matrix and a polar polymer is a domain, is superior as an electret over a blend system of a reverse structure, in which a polar polymer is a matrix and a nonpolar polymer is a domain.